SOIL ENGINEERING: TESTING, DESIGN, AND REMEDIATION phần 8

4. The cut-off from moisture intrusion 5. The stabilization of slope 6. The subdrain around the basement 13.1 METHODS OF DRAINAGE The movement of water through the soil mass is generally termed “seepage.” In engineering practice, soil is drained where it is desirable to eliminate seepage pressure or to increase the shearing strength. Drainage also is important when it is necessary to prevent seepage into the soil from exterior sources. Some of the methods of drainage are: Drainage by gravity Drainage by consolidation Drainage by desiccation Drainage by electro-osmosis Drainage by suction Of the above, the interests of the consultants are obviously in “drainage by gravity,” although soil suction on expansive soils has received increased attention. 13.1.1 PERMEABILITY The method of drainage in an excavation of a construction project depends on the slope of the excavation, the permeability of the soils, the elevation of the water table, and the available equipment. If the soil mass through which the seepage occurs is compacted fill, such as a dam or an embankment, the permeability can be reduced by the proper selection of material used. The classification of soils according to their coefficient of permeability is as shown in Table 13.1. TABLE 13.1 Coefficient of Permeability of Various Types of Soils Degree of permeability Values of k (cm/s) High Medium Low Very Low Impervious coarse gravel sand, fine sand silty sand, dry sand silt clay Over 10–3 10–3 to 10–5 10–5 to 10–7 10–7 to 10–9 less than 10–9 In most soils, the coefficient of permeability depends on the direction in which water is traveling. In the direction parallel to the bedding planes, the value can be 2 to 20 times that in the direction perpendicular to the bedding stratification. ©2000 CRC Press LLC In the Rocky Mountain area, the claystone shale is generally impermeable. However, it contains numerous seams and fissures through which water can flow easily. The effective permeability will be far greater than that of the intact materials between the cracks. Laboratory permeability tests have little meaning with respect to the actual value. Consultants are warned to be very cautious in dealing with claystone since it may be the source of many foundation distress problems. For major projects, it is advisable for the geotechnical engineers to seek advice from an experienced geologist and water engineers. Probably the most reliable method in determining the permeability of claystone bedrock is by conducting a field pump test. 13.1.2 SLOPE AND DRAIN Slope modification and provision of drainage are positive methods for improving the stability of both cut and embankment slopes. Backfill around new construction is usually not well compacted and in many cases results in negative slopes. Surface water not only cannot drain away from the structure, but also accumulates around the foundation soil. Both the architects and the geotechnical engineers tend to ignore such conditions as their contracts do not involve such construction control. For excavation drainage, methods that require pumping are commonly used. For permanent stabilization of slopes, a means of providing for gravity flow is necessary. Interception drains, ranging from farm-tile installations to elaborate forms of drain and filter constructions, are used. Special attention should be directed to the installation of roof drains. It is a common practice to connect a roof drain outlet to the horizontal drain under the building. When the connection is clogged, all the water collected from the roof will drain into the foundation soils. Such conditions will not be detected for many years. As a result, the foundation can experience excessive settlement or unexpected heave. The location of the drain system depends on the initial seepage pattern. If the drainage is for a building site, a highway, or a runway, the initial ground water condition must be established in order to determine the most suitable drain location; such an undertaking is sometimes ignored by the architect, and the entire design may be left to the contractor. Geotechnical engineers are often provided a stack of details by the architect, including the design of a door knob, but as far as the drainage detail is concerned, it is shown only by dotted lines. A typical drain consists of three components: the filter, the collector, and the disposal system. It is a misconception that the pipe in the drain system is carrying water. Actually, it is the filter or the collector that is responsible for an efficient drain system. The filter is pervious enough to permit the flow of water into the drain with little head loss and at the same time fine enough to prevent erosion of the soil into the drain. A proper filter is the key to a successful drainage system. An improper filter, which is commonly used by contractors, is one factor that contributes to foundation failure. For a filter to provide free drainage, it must be much more pervious than the soil. Figure 13.1 shows the grain-size criteria for material used as filter. ©2000 CRC Press LLC FIGURE 13.1 Grain size criteria for soils used as filter (after Sowers). For many silty and clayey soils, a well-graded concrete sand makes a fine filter. In most construction projects, not many contractors will adhere to the above spec-ification. The typical perforations in the drain pipe are 1/16 to 5/8 in. More critical than the filter design is the outlet of the drain system. In the design stage, it is assumed that there will not be any obstacle in the outlet to prevent free flow. After years of service when the surrounding environment changes, and the outlet is blocked, such a condition will not be noticed until water from the drain backs into the building. What contractors commonly refer to as a “French drain” is made of coarse gravel or crushed rock placed in a trench. Such a device is commonly employed by the contractor to take the place of a filter drain. A properly functioning “French drain” seldom exists. 13.1.3 WELL POINT AND PUMP TEST Well points are small-diameter wells that are driven or jetted into the ground. Usually they are placed in a straight line along the sides of the area to be drained and are connected at their upper ends to a horizontal suction pipe called a header, as shown in Figure 13.2. Well points are usually spaced 6 to 30 ft. They are capable of lowering the ground water from 30 to as much as 100 ft. The cost of installation of such a system is high, and unless it is for a major project such as a large dam, the system is not warranted. FIGURE 13.2 Multiple stage well point system (after Sowers). ©2000 CRC Press LLC Pump tests involve the measurement of a pumped quantity from a well together with observations of other wells of the resulting drawdown of the groundwater. A steady state is achieved when at a constant pumping rate the levels in the observation wells also remain constant. The pumping rate and the levels in two or more obser-vation wells are then noted. The coefficient of permeability obtained from the field pump tests is far more reliable than the laboratory permeability tests. The pump test is time-consuming, and its cost is high. By utilizing an on-site drilling rig and auger drills, the cost can be reduced. Pump tests should be performed on projects where water table and infiltration rates are critical. 13.2 GROUNDWATER The term groundwater is loosely defined as a continuous body of underground water in the soil void that is free to move under the influence of gravity. The water table is the upper surface of a body of ground water. It is defined as the level of water in an open hole in the ground and is the level at which the pressure in the water is zero. Groundwater is not a static body but a stream with a sloping surface that takes many shapes, depending on the structure of the soils and rocks through which it flows. 13.2.1 WATER LEVEL Geotechnical engineers commonly determine the water table level from the measurement of the water level in the exploratory boring holes. In sandy soils, such measurements are reliable. However, in clays soil or in claystone shale such readings can be misleading. Auger rotation tends to seal the surface of the clay preventing water from seeping through holes that field engineers usually log as “dry.” In fact, as soon as overnight, water can appear in the boring holes. It is therefore important to check for water at least 24 h after drilling. Local ordinances sometimes require that the drill holes must be filled immediately after drilling. In such cases, the field engineer should fully qualify in the log the conditions under which the water level is measured. The cost of drilling foundation pier holes depends a great deal on the ground-water level. The cost of drilling in wet holes, where dewatering is required, can be several times higher than drilling in the dry holes. When contractors bid the project, they base the descriptions on the field condition given in the field log. If the field log indicated “dry” holes and water is encountered, a “change of condition” will be submitted and the geotechnical engineer is blamed for the cost increase. It is amazing that the word “dry” can involve thousands of dollars. Fluctuation of the groundwater level depends on the source of the supply. When the rate of intake exceeds the rate of loss, as it does during the wet season, the water table rises. When the intake decreases, as during the dry season, or when the loss increases because of pumping for water supply or because of drainage, the water table falls. Fluctuation of the groundwater level can reach as much as 10 ft. If the time of the foundation investigation is during a low water table period and the penetration resistance of the soil is high, the foundation can suffer damage ©2000 CRC Press LLC FIGURE 13.3 Perched aquifers (after Sowers). when the water table rises and softens the soil. The problem may not emerge until many years after the structure is completed. A large percentage of foundation distress is caused by the rise of groundwater. 13.2.2 PERCHED WATER A perched water table occurs when water seeping downward is blocked by an impermeable layer of clay or silt and saturates the area above it, as shown in Figure 13.3. An impervious stratum creates a basin that may hold groundwater that is perched above the general water table. A perched water table occurs rather frequently and is well recognized by geologists and water engineers. A small amount of perched water may be drained by drilling holes through the impervious basin, allowing the water to seep downward. However, in the Rocky Mountain region where claystone bedrock is near the ground surface, the extent of the perched water table can be very extensive. It poses a great many problems to the structure’s foundation. Perched water is fed by surface water derived from precipitation and snow melt. When the area is developed, perched water is further fed by lawn watering, drain from leaking sewer lines, and other man-made sources. A perched water reservoir can be replenished by a water source as far as a mile away. The size of a perched water reservoir can vary considerably. A small reservoir can pose a seepage problem only after a prolonged wet season, while some perched water reservoirs do not dry up even during dry seasons. With the development of a perched water condition, water from the reservoir can gradually seep into the foundation soil and cause extensive damage, especially in the expansive soil area. 13.2.3 MOISTURE BARRIER Moisture barriers are used to prevent groundwater or water from other sources from seeping into the excavation. Both vertical and horizontal barriers have been used. Theoretically, vertical barriers should be more effective than horizontal barriers in minimizing seasonal drying and shrinking of the perimeter foundation soils, as well as maintaining long-term uniform moisture conditions beneath the covered area. ©2000 CRC Press LLC ... - tailieumienphi.vn